1. Field of the Invention
The present invention relates to a method for carrying out roll-bonding of medium-alloy or high-alloy steel to mild steel, which it has never been possible to achieve on an industrial scale up to the present time.
2. Description of the Prior Art
In the well-known roll-bonding process, a tip of high-grade and therefore expensive steel is applied by forging on a substrate of low-alloy and therefore inexpensive steel at a temperature of approximately 1150.degree. C. This process leads to the production of bimetallic bars also known as tipped bars or bars of composite steel or else bars of clad steel.
This roll-bonding technique is employed at the present time mainly for the production of wearing flats such as rolling-mill slides, for example, or for the manufacture of tipped industrial blades and knives such as, for example, the blades of guillotine cutters for paper, of peeling machines or slicing machines for wood.
However, although roll-bonding achieves excellent results with low-alloy steel, it proves unsatisfactory for medium-alloy or high-alloy steel tips.
In the present description, it will be understood that low-alloy steel refers to grades of steel containing a maximum percentage of 2% of alloying elements such as silicon (Si), molybdenum (Mo), tungsten (W), vanadium (V) and/or chromium (Cr) such as, for example, the grades 80WC20, 110WC10, medium-alloy or high-alloy steels refer to grades of 90MCW5 or 60WC20. It will be understood that steel containing more than 2% of alloying elements such as, for example, Z160CDV12, or the so-called high-speed steels, type 18-0-1 or 6-5-2, which are a particular case of high-alloy steels.
The reason for which roll-bonding of medium-alloy or high-alloy steel with mild steel has not been adopted on an industrial scale up to the present time is that the alloying elements (Si, Mo, W, V, Cr) cause an excessive appearance of oxides, thus resulting in very unreliable bonding of the tip to its substrate and consequently in a high reject rate.
It is for this reason that industrial blades which are commercially available at the present time and have tips of high-speed steel are manufactured by brazing with copper since copper interposed between the mild steel substrate and the high-speed steel tip ensures efficient bonding after melting and cooling.